FIG. 1 illustrates a Radio Access Network (RAN). In this example, the RAN comprises a plurality of fronthaul networks 10 and a backhaul network 20.
Each of the fronthaul networks 10 comprises a plurality of Remote Radio units, RRUs 30 (which may also be referred to as Radio Equipments REs). These REs 30 are located at respective geographical sites, and are each coupled to one or more radio antennas 40. Each RE 30 is coupled to a respective fronthaul network node 50. In this example, each fronthaul network 10 further comprises a pool of baseband units (which may be referred to as radio equipment controllers RECs), which are located at a hub network node 55. This hub node 55 may be a few kilometers away from each of the REs 30.
In order to effectively handle communication between the pool of RECs and the respective REs 30, optical WDM (wavelength-division-multiplexed) networks are considered a promising technology. This is because optical WDM networks may provide high bandwidth and scalability, low latency and also high fibre utilisation and a high level of resilience.
In an optical WDM ring network, the pool of RECs may use respective wavelengths to communicate with respective REs 30. These wavelengths are multiplexed by the hub node 55 into a downlink WDM signal which is transmitted around the ring. Each of the network nodes 50 drops only the wavelength(s) for its RE 30, and allows the other wavelengths to pass further around the ring. Each of the network nodes 50 may also add one or more respective wavelengths from its RE 30 to an uplink WDM signal which is also transmitted around the ring, to the hub node 55.
The mobile traffic from each fronthaul network 10 is aggregated by respective router cards 70 and sent via the backhaul network 20 to edge nodes 80. From the edge nodes 80, the aggregated mobile traffic is transmitted through the IP core network, which is indicated at reference numeral 90. The backhaul network 20 may, similarly, comprise an optical WDM ring network.
However, WDM optical networks use wavelength selective switches (WSSs) which, since they are based on complex free space optics, are costly and bulky devices. This has prohibited the adoption of WDM optical networks in radio access networks. In radio access networks, the capacities to be transported are lower than in transport networks. Further, the required performance level of optical switches may be lower, since, in radio access networks, optical signals may need to traverse fewer nodes and the links between the nodes may be shorter than in transport networks.
An alternative to using WSS's is to use FOADMs (fixed optical add drop multiplexers). FOADMs may comprise a cascade of thin film filters. However, since FOADMs are passive devices, they cannot be reconfigured, for example to provide protection in the event of a fault in the WDM optical network.